Explanation of the Principles Used in Single Point Acoustic Current Meters

The RCM 9 and RCM 11 are acoustic Doppler backscatter-based current meters. Its basic way of operation is to emit an acoustic pulse and measure the frequency shift of any incoming reflection. These reflections are caused by tiny particles in the water with different acoustic impedance from that of water. It does not matter if the impedance is higher (as for tiny rock parts) or lower (as for air or gasses), both will result in a reflection.

RCM 11 and RCM 9 MkII were introduced in year 2000. RCM 11 is rated to 6000 meters, while RCM 9 Mk II is rated to 1000 meters. There is a lot of customer experience with the instruments as more than 2200 RCM 9 and RCM 11s have been sold since their introduction. Many tests of the RCM 11 have also been conducted. This paper aims to answer some common questions about the acoustic Doppler backscatter principle in general and the way it is exploited by RCM 11.